1. Field of the Invention
The present invention relates in general to an impedance matching circuit, and more particularly, to an impedance matching circuit having a plurality of microstrip structures providing a lossless target signal transmission and rejecting image signals of heterodyne noise and super-heterodyne noise.
2. Description of the Prior Art
Although a wired communication system is taken for granted as a means of signal transmission for most local area networks (LAN), wireless network applications have gained popularity over the past few years. With continuing development of related technology, the changes brought about by wireless communication are gradually penetrating into almost every field of traditional communication systems. Additionally, the cost of switching from a wired signal transmission to a wireless signal transmission has been slashed due to the maturity of the related technology. Today, in order to avoid troublesome wiring problems, a wireless communication system becomes more and more crucial and is demanded by some local area signal transmissions, such as a wireless application protocol (WAP) browser of a mobile phone, which lets users connect to the Internet and access special designed services and WAP pages.
With the aim of quickly spreading wireless communication technology into every phase of office life and enhancing technological improvement, a standard must be created to ensure compatibility and reliability of signal transmission among all the related devices and systems. Therefore, wireless transmission standards were created by the Institute of Electrical and Electronics Engineers (IEEE), such as the IEEE 802.11 standard in 1997, and the newer standards of IEEE 802.11a and IEEE 802.11b created in 1999. The early standards define the specification of RF-band usages and regulate signal transmission rate. The new version standards of IEEE 802.11a and IEEE802.11b are based on the band signals of 5.8 GHz and 2.4 GHz to specify the physical layer transmission rate. All these specifications can be applied to general transmission signals in the Industrial-Scientific-Medical (ISM) bands, such as bands of 902-928 MHz, 2.4-2.4835 GHz, 5.150-5.350 GHz, and 5.725-5.850 GHz.
Please refer to FIG. 1, which shows a function block diagram of a prior art transceiver 10. The transceiver 10 is a front-end circuit to receive a low-power radio-frequency (RF) signal. Traditionally, there are various modes of processing the RF signal received by the transceiver 10, such as a heterodyne, a super-heterodyne, or a zero intermediate frequency (IF) topology. Because of a DC voltage offset, a transceiver with zero-IF topology is known to have a narrower dynamic range. Moreover, because of circuit design considerations, although a transceiver with either a heterodyne or a super-heterodyne topology is known to have a broader dynamic range, extra filters are required to get rid of unwanted image signals.
The prior art transceiver 10 in FIG. 1 comprises an antenna 11, an input circuit 13, a RF amplifier 14, a mixer 16, a local oscillator 18, an IF amplifier 20, a demodulator 22, and an output device 23. After receiving an RF signal 12 through the antenna 11, the input circuit 13 is utilized to pick up desirable signals and match the impedance between the RF amplifier 14 and the antenna 11. In addition, the input circuit 13 is able to avoid secondary radiation coming from the RF signal 12 received by antenna 11. The RF amplifier 14 is not a necessity in circuits of the transceiver 10, however, with the aid of the RF amplifier 14, the receiving performance of the transceiver 14 is improved. For instance, if the RF amplifier 14 increases the gain of the RF signal 12, the back-end circuits, such as mid-band or base-band circuits, can be easily driven by the amplified signal. Nevertheless, unwanted noises are amplified at the same time. In order to improve the signal-to-noise ratio of the circuit and avoid unwanted radiation coming from the local oscillator 18 through the antenna 11, devices having features of low noise, high forward gain and high reverse isolation are required, with GaAs hetero-junction field effect transistors (FETs) among the candidates.
The mixer 16 functions to convert the RF signal 12 into an IF signal 17 for later amplification. The operation of the mixer 16 is based on the received RF signal 12 and the oscillating signal 17 generated by the local oscillator 18. Taking advantage of a nonlinear circuit, the mixer 16 is capable of generating various kinds of signals, such as an RF signal having the same frequency as the RF signal 12, a signal having a frequency equal to the sum of the frequencies of the RF signal 12 and the oscillating signal 17, a signal having a frequency equal to the difference of the frequencies of the RF signal 12 and the oscillating signal 17, and the other high frequency harmonic signals. Using a filter, a signal having a frequency equal to the difference of the frequencies of the RF signal 12 and the oscillating signal 17 is extracted from all the harmonic signals by the mixer 16. Because it is more difficult and costs more to design a high frequency amplifier for the RF signal 12, than to design a IF frequency amplifier for the IF signal 19, the mixer 16 converts the RF signal 12 into the IF signal 19 and sends the IF signal 19 to the IF amplifier 20. Consequently, the major gain, signal selectivity of the transceiver 10 is determined by the IF circuits, which typically comprises of a channel selection filter and the IF amplifier 20. In the end, a demodulator 22, such as an envelope detector, or a frequency discriminator, is utilized to retrieve the RF signal 12 having larger power from the amplified IF signal 19 and provides the amplified RF signal 12 to drive the output device 23, such as a loudspeaker.
As described hereinbefore, the transceiver 10 generates the IF signal 19 through a signal mixing process of the oscillating signal 17 and the low-power RF signal 12 received by the antenna 11, and an amplified RF signal 12 having enough power to drive the output device 23 can be retrieved from the amplified IF signal 19 by a demodulating process. The transceiver 10 has been used in almost every kind of signal modulation systems, such as amplitude modulation (AM) systems, frequency modulation (FM) systems, single side band (SSB) modulation systems, television systems, radar systems, mobile communication systems, and wireless communication systems. The major reason for such an extensive application comes from the high-selective bandpass effect of the IF amplifier 20, which removes the undesirable band signals other than the IF signal 19.
If the transceiver 10 is operated with a heterodyne or super-heterodyne topology, there are unwanted signals having two optional frequencies produced based on the oscillating signal 17 and the RF signal 12. One unwanted signal has a frequency higher than the frequency of the RF signal 12 received by the antenna 11 and the related phenomenon is called LO high-side injection. The other unwanted signal has a frequency lower than the frequency of the RF signal 12 and the related phenomenon is called LO low-side injection.
Taking LO high-side injection for an example, if the frequency of the oscillating signal 17 is FO, the frequency of the RF signal 12 is FRF, and the frequency of the IF signal 19 is FIF, the relationship of the three frequencies can be expressed as FO=FRF+FIF. Ideally, after the signal mixing process, the IF signal 19 having a frequency equal to the difference of the frequencies of the oscillating signal 17 and the RF signal 12, that is FIF=FOxe2x88x92FRF, is the only signal passing through the IF amplifier 20. However, there is a noise signal having a frequency FI, where FI=FRF+2FIF, only partially attenuated by the input circuit 13. After the signal mixing process of the noise signal and the oscillating signal 17, the frequency of the corresponding output signal by the mixer 16 is also equal to the frequency FIF of the IF signal 19. That is to say, the noise signal having a frequency FI will also pass through the IF amplifier 20 and interfere with the desired down-converted RF signal 12. The interfering noise signal is known as the image signal and its frequency is called image frequency.
Correspondingly, taking LO low-side injection as the example, the relationship of the three frequencies among the oscillating signal 17, the RF signal 12, and the IF signal 19 can be expressed as FO=FRFxe2x88x92FIF. Ideally, after the signal mixing process, the IF signal 19 having a frequency equal to the difference of the frequencies of the RF signal 12 and the oscillating signal 17, that is FIF=FRFxe2x88x92FO, is the only signal passing through the IF amplifier 20. However, there is a noise signal having a frequency FI, where FI=FRFxe2x88x922FIF, only partially attenuated by the input circuit 13. After mixing the noise signal and the oscillating signal 17, the frequency of the corresponding signal output by the mixer 16 is also equal to the frequency FIF of the IF signal 19. The noise signal will also pass through the IF amplifier 20 and bring interference into the desired down-converted RF signal 12. As a result, it is a must for the transceiver 10 to get rid of the image signals before the amplifying process of the IF amplifier 20 to improve the circuit performance.
It is therefore a primary objective of the claimed invention to provide an impedance matching circuit having a plurality of microstrip structures providing a lossless target signal transmission and rejecting image signals of heterodyne noise and super-heterodyne noise to solve the above-mentioned problems.
According to the claimed invention, an impedance matching circuit is connected between an input circuit and an output circuit, the input circuit generating a target signal and an image signal associated with the target signal, the image signal being heterodyne noise or superheterodyne noise of the target signal. The impedance matching circuit includes a circuit board having a metal membrane which functions as a ground layer for providing a reference ground voltage and a first, second, and third microstrip circuit. The first microstrip circuit has a first microstrip line positioned on the circuit board and coupling with the metal membrane to form a first signal-coupling structure. The first microstrip line includes a first terminal connected to the input circuit and a second terminal being an open stub. The second microstrip circuit has a second microstrip line positioned on the circuit board and coupling with the metal membrane to form a second signal-coupling structure. The second microstrip line includes a first terminal being open-circuited and a second terminal connected to the output circuit. The third microstrip circuit has a third microstrip line, with a third predetermined length being determined according to a frequency of the image signal, and positioned on the circuit board and coupling with the metal membrane to form a third signal-guiding structure. The third microstrip line includes a first terminal connected to either the first microstrip line or the second microstrip line and a second terminal being open-circuited. The first, second, and third microstrip lines are conductive bars. When the target signal and the image signal are both inputted into the impedance matching circuit, the image signal will bypass through the third microstrip line toward the ground layer, and the first microstrip line couples with the second microstrip line to generate an electromagnetic coupling to pass the target signal from the first microstrip line to the second microstrip line and output the target signal to the output circuit.
These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.